The drafting of textile fiber strands of sliver, roving or the like by means of a plurality of pairs of upper and lower rollers has long been known. The pairs of rollers are typically arranged along the textile fiber strand pathway such that the peripheral speed of each pair of successive rollers is greater than the preceding pair of drafting rollers. However, the conventional drafting rollers suffer a fundamental shortcoming in that they tend to flatten the textile fiber strand in the nip between the drafting rollers during the drafting process. As the textile fiber strand is flattened, it tends to assume a relatively wider, ribbon-like shape which results in the opposing side or selvedge fibers being attenuated more than the fibers in the medial portion of the textile fiber strand. This, of course, introduces a certain amount of undesirable non-uniformity in the resulting drafted fiber strand which is produced by the drafting apparatus.
It has been found that, as the textile fiber strand is compressed between a pair of drafting rollers into the ribbon-like configuration, the fibers in the medial portion of the strand are subjected to a greater relative pressure than those on the opposing edges or selvedges of the strand. The variation in pressure to which the fibers are subjected at the nip of each pair of drafting rollers introduces non-uniformities into the resulting drafted strand since the fibers in the selvedges of the strand tend to be released from the nip of each pair of drafting rollers prior to the fibers in the medial portion of the flattened ribbon-like strand. Therefore, the need for a drafting apparatus utilizing a plurality of pairs of drafting rollers which are able to exert a more uniform pressure on the fibers in the textile fiber strand during the drafting process so as to result in a more uniform drafted strand has long been recognized in the textile art.
To date, the value of the use of a tongue and groove type roller assembly for compressing a textile fiber strand prior to feeding it to drafting rollers has been appreciated. U.S. Pat. No. 4,551,887 to Murata discloses a draft roll assembly for use with a pneumatic high speed spinning frame which comprises one or more pairs of tongue and groove compression rollers to receive a sliver strand and compress it prior to the sliver strand being fed to conventional draw rollers. The tongue and groove rollers are intended to compress the sliver strand while containing its width-wise expansion so that its density will be increased at drafting in order to impart improved drafting efficiency and higher quality yarn spun from the drafted strand. Of particular note, lines 23-46 of column 2 teaches that while it is possible to arrange a plurality of pairs of tongue and groove compressing rollers upstream of the drafting rollers, any drafting phenomenon is undesirable and is to be avoided.
U.S. Pat. No. 4,489,461 to Toyoda also discloses a drafting system for drafting textile sliver prior to the fiber being spun into yarn. The drafting system utilizes a first pair of tongue and groove rollers to compact the sliver into a reduced cross section therebetween and a subsequent pair of conventional drafting rollers rotating at a much higher peripheral speed than the tongue and groove rollers. In this fashion, the drafting system is able to obviate the necessity for utilizing conventional drafting aprons to transfer the fiber strand from one pair of drafting rollers to the next pair. Also of interest, U.S. Pat. No. 4,539,729 to Rieter discloses a tongue and groove roller pair for the continuous compression of a fiber sliver wherein the sliver feed funnel is positioned within a wedge-shaped gap which is bounded by the overlapping zone of the side surfaces of the interacting rollers.
However, applicant does not believe that the benefits of continuously drafting a sliver strand in a condensed and uniform configuration between a plurality of pairs of tongue and groove type drafting rollers in order to improve the distribution of forces on the individual fibers within the strand and therefore the resulting uniformity of the drafted fiber strand has been appreciated in the art to this date.